Light-emitting diodes (LEDs) are widely used as solid-state light sources. Compared to conventional incandescent light lamps or fluorescent light tubes, LEDs have advantages such as lower power consumption and longer lifetime, and therefore LEDs gradually replace the conventional light sources and are applied to various fields such as traffic lights, back light modules, street lighting, and biomedical device.
FIG. 24 is a cross-sectional diagram showing a conventional vertical cavity surface emitting laser (VCSEL). A vertical cavity surface emitting laser (VCSEL) is capable of emitting coherent light in a direction perpendicular to an active region. VCSEL comprises a structure having a substrate 300, a pair of DBR stacks 200,210 on the substrate 300 interposing an active region 230 where electrons and holes combine to generate light. A first electrode 240 and a second electrode 250 are provided for an electrical current to be injected into the active region to generate light, and the light is exited from an aperture on an upper surface of the VCSEL.
The vertical cavity surface emitting laser may have an undercut aperture 260 in one of the DBR stacks 210. The undercut aperture 260 is formed by selectively etched away a periphery part of one of layers in the DBR stack 210, and thus an air gap having relatively low conductivity compared to the conductivity of the other layers is formed in the DBR stack 210.